Solenoid-actuated control valves, herein after referred to as control valves, are well known to control the flow and/or pressure of a fluid. In many applications, it may be desirable that the flow and/or pressure output of the control valve be proportional to an electric current supplied to a solenoid of the control valve. In a common control valve arrangement, the electric current supplied to the solenoid of the control valve affects the position of a supply valve member and/or an exhaust valve member relative to a supply valve seat and an exhaust valve seat respectively. The position of the supply valve member relative to the supply valve seat and/or the position of the exhaust valve member relative to the exhaust valve seat affects the fluid flow and/or pressure leaving the control valve. It is therefore important that the position of the valve members relative to the valve seats for a given electric current supplied to the solenoid does not change during the life of the control valve because if the position of the valve members relative to the valve seats is not as is expected, the flow and/or pressure leaving the control valve may not be the desired magnitude.
The solenoid of the control valve is typically enclosed in a housing that is cylindrical and made of metal. An example of such a control valve is shown in US Patent Application Publication No. US 2007/0138422 A1. During manufacturing of the control valve, at least one end of the housing is open to allow components of the solenoid to be placed within the housing. After all of the components have been placed within the housings, a cover may be placed over the open end, and the housing may be crimped or folded over the cover to retain the cover to the housing. When the housing is crimped to retain the cover, an axial load is placed on the components within the housing and the axial load on the components within the housing is maintained by the crimp connection. However, this axial load from the crimp is known to be transmitted through plastic components, such as the spool (also known as a bobbin or coil former) around which the coil of the solenoid is wound. Over time, this crimp load may cause the plastic components to creep (i.e. change in shape and position), thereby changing the position of the valve members relative to the valve seats for a given a given electric current supplied to the solenoid. As a result, the desired flow and/or pressure leaving the control valve may not be the desired magnitude for a given electric current supplied to the solenoid.
One way to address the effects of creep of the plastic components and the changing of position of the valve members relative to the valve seats over time is to use closed loop feedback. In this arrangement, the actual flow and/or pressure leaving the control valve is measured with a sensor. The sensor sends a signal indicative of the flow and/or pressure to a controller. If the signal indicates that the flow and/or pressure is not the desired magnitude, the controller can alter the electric current supplied to the solenoid until the desired flow and/or pressure reaches the desired magnitude. In this way, the effects of creep of plastic components can be overcome. However, using closed loop feedback increases the cost and complexity of the system, for example by the addition of sensors, wiring, and software.
What is needed is a control valve in which the flow and/or pressure leaving the control valve does not change over time for a given magnitude of electric current used to actuate the control valve. What is also needed is a control valve which is not affected by creeping of plastic components of the solenoid over time.